Observatory Base Structure Page

Base Structure Design

The base structure frame is a 10 sided regular polygon, 1 meter high with a 1.62 meter circumradius (below left photo)The base structure is built around the telescope pier, without direct contact between the pier and observatory. The basic design was modeled after a photo on  Clement's Observatory Website.  I bent 6 mm plywood around the 10 sided frame, giving a pretty good approximation to a circular base structure. The dome is 3.36 meter in diameter and the base structure is slightly less in diameter, allowing 5-6 cm of dome overhang for better rain run off. The below right photo shows the base structure (7 years old) after 2015 dome replacement. 

Base Structure Construction

The top horizontal cross supports (3.8 cm x 5.7 cm x 1 m) form a 72 degree angle with the radius. I set my miter saw at 18 degrees and cut all cross braces and exterior floor joists.  The 10 upright posts (7.2 cm x 3.6 cm x 1 m) and all floor joists (4.5 cm x 9.5 cm) rest on the masonry blocks. The cross bracing between the 10 upright posts and the perimeter floor joists are 3.8 cm x 5.7 cm x 55 cm, with a 45 degree bottom miter and a 45 degree x 18 degree compound top miter.  The floor sits on the 10 radial floor joists and the 10 perimeter floor joists (below left). I installed two additional concentric rings of floor joists, each set 40 cm on center. The below left and right diagrams show the base structure floor joist plan and side plan, respectively. The base structure is connected with M8 exterior bolts to 4 pressure treated posts, each anchored in concrete and located around the outer perimeter. If I ever want to move the structure, it can then be easily disconnected from the posts without removing the flooring.

Most home observatories have an additional plywood base ring above the top braces to increase stability. I fabricated a 3 layer thick plywood ring to support the dome (dome ring), but I wanted to eliminate using an additional plywood base ring on top of the top braces. My solution was to strengthen the top braces by adding corner supports at all 10 uprights. The corner supports are just scraps of 3.8 cm x 5.7 cm wood, cut to a 144 deg. angle, that fit into the internal angles where the top braces meet (below photo). This greatly strengthened the top braces and eliminated the need for an additional plywood base ring.                                                                                                            


The floor required 80 linear meters of 25 mm x 12 cm pine boards and cost about 50% as much as 8 m2 of plywood. 25 mm pine was thick enough for most of the flooring, with the exception of the three longest boards at the outer perimeter. I installed 10 additional cross joists perpendicular to the outer perimeter, which gave satisfactory support to the 25 mm pine.  There is 3-4 mm spacing between boards, to allow for expansion. The below photo shows the finished observatory floor. All surfaces of the floor boards were treated with an exterior oil based water sealant and secured with 55 mm galvanized nails.

Exterior Siding and Solar Powered Ventilation Fan

The exterior siding is 6 mm thick interior plywood, treated annually with an oil based water sealant. The 6 mm plywood easily bends around the 10 sided frame, giving a pretty good approximation to a circular base structure. The plywood is fastened with galvanized screws. The below left photo shows the base structure exterior during dome replacement (2015), when the exterior siding was 7 years old. The plywood receives an annual treatment with oil based water sealant and has held up very well. After 7 years, there is only a single water damaged area (directly below the padlock). Next summer (2016), I plan to replace the plywood with exterior metal siding.

During 2014 I noticed significantly more mould inside the observatory than in previous years. I installed a solar powered ventilation fan to increase airflow and inhibit mould growth (below right photo). The solar fan is mounted inside a PVC pipe and draws approx. 40 cubic meters of air per hour. I built an enclosure around the solar panel to provide extra protection from wind and rain. The enclosure is just two plywood side plates with a transparent polycarbonate cover. I also added some netting over the fan inlet to keep insects out of the observatory. 

Dome Bearings

My first dome was covered with Bituwell plates, which are bitumen impregnated sheets of inorganic-organic fibers (5.4 kg per 0.93 m x 2 m x 3 mm sheet). The first dome was very light and only required 10 dome bearings rollers, each rated for a 25 kg load (250 kg total). I replaced the original dome during 2015 with a heavier dome. I added a second ring of white wheels to the base structure, giving a total of 20 wheels with a maximum limit of 450 kg (above left photo).

Centering Bearings

The centering bearings serve two functions: they keep the dome centered on its axis of rotation and prevent the dome from lifting off of the dome bearings. The centering bearings are constructed from ribbed brackets, small roller wheels, and left over floor joist scraps (4.5 cm x 9.5 cm).


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